Conventional solenoid fuel injectors struggle to inject small volumes with good shot-to-shot repeatability.
For small injections, conventional peak and hold current-driven implementations provide short current pulses to move an armature, which lifts a valve from a valve seat with minimal or no dwell time at the largest lift. In this approach (often referred to as the “ballistic” regime), shot-to-shot repeatability of small injection volumes is poor. This is due to the build variation between injectors and the inability to control the partial valve lifts achieved and the opening and closing flight times, thus, variations in fuelling. Furthermore, the fuel valve will hit the open stop as the pulse width applied to the injector is increase. Normally injector operate in this linear region where the fuel valve position is fully opened to an open stop which enable linear injection fuel quantities with respect to pulse width applied to be delivered. If the open stop is hit, then this causes a rebound which will reduce the fuelling. Therefore, at pulse widths just above the minimal pulse width at which the open stop is first hit solenoid injectors often exhibit reduced injection quantities as the actuation pulse width is increased. These fuelling problems can result in the optimal air/fuel mix not always being delivered which in turn can cause less efficient combustion and, thus, increase emissions. In addition smooth running of the engine at low loads is impaired.
Non-linearity in fuel volume as a function of pulse width can make closed-loop control difficult. In addition, impact of parts of the armature or pintle with an open stop can result in noise which can have a detrimental impact on driving experience. Furthermore, drift in injection volumes can be caused by wear at the open stop.
Some injectors use a piezoelectric actuator. Such injectors are often referred to as “piezo injectors”.
A piezo injector can provide predictable partial lift of the valve and may not use an open stop. The piezo partial lift enables repeatable small fuel deliveries to be achieved with good shot-to-shot repeatability. However, piezo-electric actuated injectors are expensive due to the cost of the actuators and the cost of associated components such as thermal compensators and/or systems to magnify the stroke of the piezo actuator. Therefore, attention is being directed to solenoid injectors to which provide similar performance to piezo injectors, but at lower cost.
EP 2 499 646 A1 describes a short-travel solenoid actuator which comprises at least one pole piece, an armature, an electromagnet coil arranged, in response to energisation, to actuate the armature between first and second positions. A permanent magnet is positioned and orientated so as to latch the armature in the first and second positions when the armature is in the first and second positions respectively. A spring is arranged to bias the armature.
EP 2 721 333 discloses a solenoid actuator which comprises an armature, pole piece(s) and electromagnet coil(s) arranged, in response to energisation, to cause travel of the armature between first and second positions along a direction of travel. The solenoid actuator includes permanent magnet(s) positioned and orientated for latching the armature in at least the first position when the armature is in the first position and spring(s) arranged to bias the armature. The solenoid actuator can be operated to provide partial lift
U.S. Pat. No. 8,166,953 B2 describes a solenoid valve used as part of a fuel injector and controlled to provide partial lift of the armature.
EP 1 571 679 B1 describes a voltage-driven solenoid valve in which a drive waveform is adaptively obtained from data from the coil current feedback and is continually adjusted during use to compensate for changing drive conditions.
U.S. Pat. No. 8,681,468 B2 describes a solenoid valve in current and voltage are monitored and are used to adaptively define the drive waveform in order to keep drive levels and times to a minimum and reduce wasted energy in the system.
U.S. Pat. No. 5,161,083 A describes rotary and linear proportional solenoid actuators capable of partial lift in which feedback from a position sensor is used as part of closed-loop control of the drive current to maintain the lift position of the actuator.
U.S. Pat. No. 7,984,706 B2 describes a fuel injector with a two-coil solenoid actuator which uses current driven through each coil independently in order to decelerate the armature and negative current used to latch the armature to each pole thereby reducing bounce.
U.S. Pat. No. 7,726,276 B2 describes closed-loop control of direct injection systems using a cylinder-selective lambda sensor to control injection quantities.
EP 2 587 034 A1 describes a fuel injector with a solenoid actuator. The voltage (or current) through the coil is monitored, at least in the closing phase of the actuator, and used to determine the fuel pressure delivered by the high-pressure fuel pump. This enables the fuel pump to be supervised and removes the need for a separate pressure sensor.
EP 1 369 571 B1 describes a method of controlling a solenoid fuel injector to provide an increased dynamic range (i.e. the range over which injector delivers a quantity of fuel that varies linearly with open time to within a predetermined margin).